.TH PRECONV 1 "7 March 2023" "groff 1.22.4"
.SH NAME
preconv \- convert encoding of input files to something GNU troff \
understands
.
.
.\" Save and disable compatibility mode (for, e.g., Solaris 10/11).
.do nr preconv_C \n[.C]
.cp 0
.
.
.\" ====================================================================
.\" Legal Terms
.\" ====================================================================
.\"
.\" Copyright (C) 2006-2018 Free Software Foundation, Inc.
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of
.\" this manual under the conditions for verbatim copying, provided that
.\" the entire resulting derived work is distributed under the terms of
.\" a permission notice identical to this one.
.\"
.\" Permission is granted to copy and distribute translations of this
.\" manual into another language, under the above conditions for
.\" modified versions, except that this permission notice may be
.\" included in translations approved by the Free Software Foundation
.\" instead of in the original English.
.
.
.\" ====================================================================
.SH SYNOPSIS
.\" ====================================================================
.
.SY preconv
.OP \-dr
.OP \-D default_encoding
.OP \-e encoding
.RI [ file
\&.\|.\|.\&]
.
.SY preconv
.B \-h
.SY preconv
.B \-\-help
.YS
.
.SY preconv
.B \-v
.SY preconv
.B \-\-version
.YS
.
.
.\" ====================================================================
.SH DESCRIPTION
.\" ====================================================================
.
.B preconv
reads
.I files
and converts its encoding(s) to a form GNU
.BR troff (1)
can process, sending the data to standard output.
.
Currently, this means ASCII characters and \[oq]\e[uXXXX]\[cq]
entities, where \[oq]XXXX\[cq] is a hexadecimal number with four to
six digits, representing a Unicode input code.
.
Normally,
.B preconv
should be invoked with the
.B \-k
and
.B \-K
options of
.BR groff .
.
.
.\" ====================================================================
.SH OPTIONS
.\" ====================================================================
.
Whitespace is permitted between a command-line option and its argument.
.
.
.TP
.B \-d
Emit debugging messages to standard error (mainly the used encoding).
.
.TP
.BI \-D encoding
Specify default encoding if everything fails (see below).
.
.TP
.BI \-e encoding
Specify input encoding explicitly, overriding all other methods.
.
This corresponds to
.BR groff 's
.BI \-K encoding
option.
.
Without this switch,
.B preconv
uses the algorithm described below to select the input encoding.
.
.TP
.B \-\-help
.TQ
.B \-h
Print a help message and exit.
.
.TP
.B \-r
Do not add \&.lf requests.
.
.TP
.B \-\-version
.TQ
.B \-v
Print the version number and exit.
.
.
.\" ====================================================================
.SH USAGE
.\" ====================================================================
.
.B preconv
tries to find the input encoding with the following algorithm.
.
.IP 1.
If the input encoding has been explicitly specified with option
.BR \-e ,
use it.
.
.IP 2.
Otherwise, check whether the input starts with a
.I Byte Order Mark
(BOM, see below).
.
If found, use it.
.
.IP 3.
Otherwise, check whether there is a known
.I coding tag
(see below) in either the first or second input line.
.
If found, use it.
.
.IP 4
Finally, if the
.B uchardet
library
(an encoding detector library available on most major distributions)
is available on the system, use it to try to detect the encoding of the file.
.
.IP 5.
If everything fails, use a default encoding as given with option
.BR \-D ,
by the current locale, or \[oq]latin1\[cq] if the locale is set to
\[oq]C\[cq], \[oq]POSIX\[cq], or empty (in that order).
.
.
.PP
Note that the
.B groff
program supports a
.I \%GROFF_ENCODING
environment variable which is eventually expanded to option
.BR \-k .
.
.
.\" ====================================================================
.SS "Byte Order Mark"
.\" ====================================================================
.
The Unicode Standard defines character U+FEFF as the Byte Order Mark
(BOM).
.
On the other hand, value U+FFFE is guaranteed not be a Unicode character at
all.
.
This allows detection of the byte order within the data stream (either
big-endian or little-endian), and the MIME encodings \%\[oq]UTF-16\[cq]
and \%\[oq]UTF-32\[cq] mandate that the data stream starts with U+FEFF.
.
Similarly, the data stream encoded as \%\[oq]UTF-8\[cq] might start
with a BOM (to ease the conversion from and to \%UTF-16 and \%UTF-32).
.
In all cases, the byte order mark is
.I not
part of the data but part of the encoding protocol; in other words,
.BR preconv 's
output doesn't contain it.
.
.
.PP
Note that U+FEFF not at the start of the input data actually is
emitted; it has then the meaning of a \[oq]zero width no-break
space\[cq] character \[en] something not needed normally in
.BR groff .
.
.
.\" ====================================================================
.SS "Coding Tags"
.\" ====================================================================
.
Editors which support more than a single character encoding need tags
within the input files to mark the file's encoding.
.
While it is possible to guess the right input encoding with the help of
heuristic algorithms for data which represents a greater amount of a natural
language, it is still just a guess.
.
Additionally, all algorithms fail easily for input which is either too short
or doesn't represent a natural language.
.
.
.PP
For these reasons,
.B preconv
supports the coding tag convention (with some restrictions) as used by
.B "GNU Emacs"
and
.B XEmacs
(and probably other programs too).
.
.
.PP
Coding tags in
.B "GNU Emacs"
and
.B XEmacs
are stored in so-called
.IR "File Variables" .
.
.B preconv
recognizes the following syntax form which must be put into a troff comment
in the first or second line.
.
.RS
.PP
\-*\-
.IR tag1 :
.IR value1 ;
.IR tag2 :
.IR value2 ;
\&.\|.\|.\& \-*\-
.RE
.
.
.PP
The only relevant tag for
.B preconv
is \[oq]coding\[cq] which can take the values listed below.
.
Here an example line which tells
.B Emacs
to edit a file in troff mode, and to use \%latin2 as its encoding.
.
.RS
.PP
.EX
\&.\[rs]" \-*\- mode: troff; coding: latin-2 \-*\-
.EE
.RE
.
.
.PP
The following list gives all MIME coding tags (either lowercase or
uppercase) supported by
.BR preconv ;
this list is hard-coded in the source.
.
.RS
.PP
.ad l
\%big5, \%cp1047, \%euc-jp, \%euc-kr, \%gb2312, \%iso-8859-1,
\%iso-8859-2, \%iso-8859-5, \%iso-8859-7, \%iso-8859-9, \%iso-8859-13,
\%iso-8859-15, \%koi8-r, \%us-ascii, \%utf-8, \%utf-16, \%utf-16be,
\%utf-16le
.ad
.RE
.
.
.PP
In addition, the following hard-coded list of other tags is recognized
which eventually map to values from the list above.
.
.RS
.PP
.ad l
\%ascii, \%chinese-big5, \%chinese-euc, \%chinese-iso-8bit, \%cn-big5,
\%\%cn-gb, \%cn-gb-2312, \%cp878, \%csascii, \%csisolatin1,
\%cyrillic-iso-8bit, \%cyrillic-koi8, \%euc-china, \%euc-cn,
\%euc-japan, \%euc-japan-1990, \%euc-korea, \%greek-iso-8bit,
\%iso-10646/utf8, \%iso-10646/utf-8, \%iso-latin-1, \%iso-latin-2,
\%iso-latin-5, \%iso-latin-7, \%iso-latin-9, \%japanese-euc,
\%japanese-iso-8bit, \%jis8, \%koi8, \%korean-euc, \%korean-iso-8bit,
\%latin-0, \%latin1, \%latin-1, \%latin-2, \%latin-5, \%latin-7,
\%latin-9, \%mule-utf-8, \%mule-utf-16, \%mule-utf-16be,
\%mule-utf-16-be, \%mule-utf-16be-with-signature, \%mule-utf-16le,
\%mule-utf-16-le, \%mule-utf-16le-with-signature, \%utf8, \%utf-16-be,
\%utf-16-be-with-signature, \%utf-16be-with-signature, \%utf-16-le,
\%utf-16-le-with-signature, \%utf-16le-with-signature
.ad
.RE
.
.
.PP
Those tags are taken from
.B "GNU Emacs"
and
.BR XEmacs ,
together with some aliases.
.
Trailing \%\[oq]-dos\[cq], \%\[oq]-unix\[cq], and \%\[oq]-mac\[cq]
suffixes of coding tags (which give the end-of-line convention used in
the file) are stripped off before the comparison with the above tags
happens.
.
.SS "Iconv Issues"
.B preconv
by itself only supports three encodings: \%latin-1, cp1047, and \%UTF-8;
all other encodings are passed to the
.B iconv
library functions.
.
At compile time it is searched and checked for a valid
.B iconv
implementation; a call to \[oq]preconv \-\-version\[cq] shows whether
.B iconv
is used.
.
.
.\" ====================================================================
.SH BUGS
.\" ====================================================================
.
.B preconv
doesn't support
.I "local variable lists"
yet.
.
This is a different syntax form to specify local variables at the end of a
file.
.
.
.\" ====================================================================
.SH "SEE ALSO"
.\" ====================================================================
.
.BR groff (1)
.br
the
.B "GNU Emacs"
and
.B XEmacs
info pages
.
.
.\" Restore compatibility mode (for, e.g., Solaris 10/11).
.cp \n[preconv_C]
.
.
.\" Emacs setting
.\" Local Variables:
.\" mode: nroff
.\" End:
.\" vim: set filetype=groff:
